koyapete - petrolem eng. - pete 203: properties of...

PETE 203: Properties of oil

Prepared by: Mr. Brosk Frya Ali

Koya University, Faculty of Engineering,

Petroleum Engineering Department

2013 – 2014

Lecture no. (2): Crude oil –chemistry and composition

The crude oil mixture is composed of the following groups:

1. Hydrocarbon compounds

2. Non hydrocarbon compounds

3. Organo-metallic compounds and inorganic salts

5. Crude oil Chemistry

The chemical properties (physical also) of crude oil depends

on its unique mixture of molecule.

5.1 Hydrocarbon compounds:-

Hydrocarbons may be gaseous, liquid, or solid at normal temperature

and pressure, depending on the number and arrangement of the

carbon atoms in their molecules.

Those with up to 4 carbon atoms are gaseous; those with 20 or more

are solid; those in between are liquid.

Properties of oil Lecture NO.2

All hydrocarbon classes are present in the crude mixture, except

alkenes and alkynes.

Alkanes (paraffins) are saturated hydrocarbon. They have only single

bonds such as CH4, C2H6, C3H8, with general formula CnH2n+2. The naming according to IUPAC ended with ane which means single.

Alkenes (Olefins) are unsaturated hydrocarbon. They have double

bonds such as Ethene (Ethylene) C2H4, propene C3H6, butene C4H8

with general formula CnH2n.

The naming according to IUPAC ended with ene which means double.

Alkynes are unsaturated hydrocarbon. They have triple bonds such as

Ethyne (acetylene) C2H2, propyne C3H6, with general formula CnH2n - 2.

The naming according to IUPAC ended with yne which means triple.

The principal constituents of most crude oils are hydrocarbon

compounds.


5.1.1 Alkanes (paraffins)

The following is a brief description of the different hydrocarbon

classes found in all crude oils.

Methane, Ethane, propane and butane are gaseous hydrocarbons at

ambient temperatures and atmospheric pressure. They are usually

found associated with crude oils in a dissolved state.

A branched alkane with the same number of carbons and hydrogen

as an n-alkane is called an isomer.

Normal alkanes (n-alkanes, n-paraffins) are straight-chain

hydrocarbons having no branches.

Branched alkanes are saturated hydrocarbons with an alkyl

substituent or a side branch from the main chain.


For example, butane (C4H10) has two isomers, n-butane and

2-methyl propane (isobutane).

As the molecular weight of the hydrocarbon increases, the

number of isomers also increases.


Pentane (C5H12) has three isomers; hexane (C6H14) has five.

The following shows the isomers of hexane:

Crude oils contain many short, medium, and long-chain normal

and branched paraffins.


5.1.2 Cycloparaffins (Naphthenes)

Saturated cyclic hydrocarbons, normally known as naphthenes,

are also part of the hydrocarbon constituents of crude oils. Their

ratio, however, depends on the crude type.

The cycloalkanes contain only single bonds, and have the

general formula CnH2n.

Cyclomethane and cycloethane obviously cannot exist, but

cyclopropane, cyclobutane, cyclopentane etc can exist.


Cyclohexanes, substituted cyclopentanes, and substituted

cyclohexanes are important precursors for aromatic

hydrocarbons.

The examples shown here are for three naphthenes of special

importance.

The lower members of naphthenes in crude oil are

cyclopentane, cyclohexane, and their mono-substituted

compounds.


If a naphtha fraction contains these compounds, the first two

can be converted to benzene C6H6, and the last compound can

dehydrogenate to toluene C7H8 during processing.

Heavier petroleum fractions such as kerosine and gas oil may

contain two or more cyclohexane rings bonded through two

neighboring carbons.

The content of cycloparaffins in petroleum varies up to 60% of

the total hydrocarbons.


5.1.3 Aromatic Compounds

Aromatic hydrocarbons are unsaturated hydrocarbons which

have one or more benzene ring, to which hydrogen atoms are

attached according to the formula CnHn.

Benzene ring is hexagonal ring arrangement found in benzene

and other aromatic compounds, consisting of six carbon atoms

with alternating single and double bonds between them, and

with each carbon atom bonded to a hydrogen atom, or to other

atoms or groups of atoms in derivatives of benzene.


Toluene (C7H8) and xylene (C8H10) are also mononuclear aromatic

compounds found in variable amounts in crude oils.

Benzene, toluene (C7H8), and xylenes (C8H10) (BTX) are

important petrochemical intermediates as well as valuable

gasoline components.

Separating BTX aromatics from crude oil distillates is not

feasible because they are present in low concentrations.

Enriching a naphtha fraction with these aromatics is possible

through a catalytic reforming process.


5.2 Non-hydrocarbon

Compounds

The table shows the percentage

by weight of hydrocarbons in the

crude oils

Table 2.1 Composition by weight of hydrocarbons in the crude oil

Hydrocarbons Average Paraffins 30 %

Napthenes 49 % Aromatics 15 % Asphaltic 6 %

The presence of these impurities is harmful and may cause

problems to certain catalytic processes. Fuels having high sulfur

and nitrogen levels cause pollution problems in addition to the

corrosive nature of their oxidization products.

Various types of non-hydrocarbon compounds occur in crude

oils and refinery streams. The most important are the organic

sulfur, nitrogen, and oxygen compounds. Traces of metallic

compounds are also found in all crudes.


5.2.1 Sulfur Components

Sulfur in crude oils is mainly present in the form of organo-sulfur

compounds. Hydrogen sulfide is the only important inorganic

sulfur compound found in crude oil.

Its presence, however, is harmful

because of its corrosive nature and

it can kill an operator in 10 seconds

(at concentration 1000 ppm)

Fortunately sulphides have a highly obnoxious (horrible) smell

which gives some warning of their danger.


Acidic sulfur compounds are the thiols (mercaptans). Thiophene,

sulfides, and disulfides are examples of non-acidic sulfur

compounds found in crude fractions.

Organosulfur compounds may generally be classified as acidic

and non-acidic.

If both of the two hydrogen atoms are replaced by hydrocarbon

groups, the compound is called a sulfide or thioether.

Mercaptans are acidic sulfur compounds .

If one of the hydrogen atoms is replaced by a hydrocarbon group,

the compound is called a mercaptan or thiol. Such compounds are

formed during the distillation of crude oils.


Examples of some sulfur compounds from the two types are:


Sour crudes contain a high percentage of hydrogen sulfide.

Because many organic sulfur compounds are not thermally

stable, hydrogen sulfide is often produced during crude

processing.

High-sulfur crudes are less desirable because treating the

different refinery streams for acidic hydrogen sulfide increases

production costs.

The presence of sulfur compounds in finished petroleum

products often produces harmful effects.

In addition, mercaptans in hydrocarbon solution cause the

corrosion of copper and brass in the presence of air.

For example, in gasoline, sulfur compounds promote corrosion

of engine parts.


Gasoline with a sulfur content between 0.2 and 0.5% has been

used without obvious harmful effect.

In diesel fuels, sulfur compounds increase wear and can

contribute to the formation of engine deposits.

Most sulfur compounds are removed from petroleum streams

through hydro-treatment processes (Hydrodesulphurization)

In this process hydrogen sulfide is produced and the

corresponding hydrocarbon released. Hydrogen sulfide is then

absorbed in a suitable absorbent and recovered as sulfur.


5.2.2 Nitrogen Components

Organic nitrogen compounds occur in crude oils either in a simple

heterocyclic form as in pyridine (C5H5N) and pyrrole (C4H5N), or

in a complex structure as in porphyrin.

Porphyrins are a group of organic compounds which occur in

nature. One of the best known is heme, the pigment in red blood

cells.


Nitrogen compounds in crude oils are complex and distillation

may give rise to nitrogen compounds.

The nitrogen content in most crudes is very low and does not

exceed 0.1 % .In some heavy crudes, however, the nitrogen

content may reach up to 0.9 % wt.

Nitrogen compounds are more thermally stable than sulfur

compounds and accordingly are concentrated in heavier

petroleum fractions and residues.

Nitrogen has to be removed from crude oil because

• Nitrogen impurities in hydrocarbon fuels have a severe

environmental impact resulting from the contribution

nitrogen oxides (produced during combustion) to acid rain;

• Nitrogen impurities are effective catalyst poisons that slow

down the processing of crude oil.


Nitrogen compounds are removed by hydrodenitrogenation to

ammonia. For example, pyridine is denitrogenated to ammonia

and pentane:

Nitrogen compounds in crudes may generally be classified into

basic and non-basic categories.

Basic nitrogen compounds are mainly those having a pyridine

ring, and the non-basic compounds have a pyrrole structure.

Both pyridine and pyrrole are stable compounds due to their

aromatic nature.


The following are examples of organic nitrogen compounds.


5.2.3 Oxygen Components

Oxygen compounds in the crude oils are more complex than the

sulfur types. However; their presence in petroleum streams is

not poisonous to processing catalysts.

Many of oxygen compounds found in crude oils are weakly

acidic. They are carboxylic acids -C(=O)OH or -COOH,

cresylic acid, phenol, and naphthenic acid.

Naphthenic acids are mainly cyclopentane and cyclohexane

derivatives having a carboxylic (-COOH).

Cresylic acid is any of several acids derived from petroleum

and coal tar that boil above 204°C, contain varying amounts of

xylene and cresol.

A cresol molecule has a methyl group substituted onto phenol

molecule.


The total acid content of most crudes is generally low, but may

reach as much as 3%, as in some California crudes

Naphthenic acids in the naphtha fraction have a special

commercial importance and can be extracted by using dilute

caustic solutions.

Non acidic oxygen compounds such as esters, ketones, and

amide are less valuable than acidic compounds.

The following shows some of the oxygen compounds commonly

found in crude oils.


Acidic Oxygen

Compounds

Non-Acidic

Oxygen

Compounds


5. 3 Metallic Components

Many metals occur in the crude oils. Some of the more

abundant are sodium (Na), calcium (Ca), magnesium (Mg),

aluminum (Al), iron (Fe), vanadium (V) and nickel (Ni).

They are present either as inorganic salts, such as sodium and

magnesium chloride, or in the form of organometallic

compounds, such as those of nickel and vanadium.

Calcium and magnesium can form salts or soaps with

carboxylic acids. These compounds acts as emulsifiers and their

presence is undesirable.

Although metals in crudes are found in trace amounts, their

presence is harmful and should be removed.


When crude oil is processed sodium and magnesium chlorides

produce hydrochloric acids, which is very corrosive.

Desalting crude oils is a necessary step to reduce these salts.

Vanadium and nickel are poisons to many catalysts and should

be reduced to very low levels.

Solvent extraction processes used to reduce the

concentration of heavy metals in petroleum residues.


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